The present invention relates to a method and apparatus for continuously separating fine or small particles contained in a fluid pulp or slurry according to density or specific gravity.
Elutriation, or rising current classification, is a method that is commonly used to separate particles in a pulp according to their settling rate. If particles of different size but of near equal density are placed in a container of static fluid, the larger particles will settle faster than the smaller particles. Conversely, if particles of near equal size but of different density are placed in a container of static fluid, the more dense particles will settle faster. If the fluid in the container is caused to flow upward through a downward settling stream of particles, the various settling rates are reduced by the velocity of the flow. Particles whose settling rates are less than the velocity of the fluid flow move upwardly and are separated from the particles whose settling rate is greater than the velocity of the flow. If particles are allowed to collect on a horizontal screen or sieve and subjected to an upward fluid flow that has a velocity that is near but lower than their settling rates, the particles will be agitated or fluidized by the flowing fluid and eventually reach equilibrium with the highest settling rate particles located nearest the screen or fluid source and the lowest settling rate particles located furthest from it. Such sorting processes are accentuated and accelerated when modified by additional forces generated by operation in a centrifuge.
Heavy media, or Sink-Float, Separation, as defined in Taggart, A Handbook of Mineral Dressing, John Wiley & Sons, Inc., NY, N.Y., 1945, page 11-104, is "subjection of a mixture of solid particles of different specific gravities to the buoyant actions of a quiescent body of fluid characteristics, having such density that it will float the lighter solid particles while the heavier sink, gravity being the only impelling force." In more simplified terms, it involves suspending the solid particles, which are to be separated according to density, in a liquid, the liquid having a density between that of the light particles and the heavy particles that are to be separated. For example, wood and rock may be separated by placing the mixture of wood particles, with a specific gravity of less than one, and rock particles, with a specific gravity greater than one, in a quiescent pool of water. The wood floats and the rocks sink, and the two materials are thus separated according to their density.
Heavy media separation relies on the "resultant density" of the materials to be separated. The resultant density is simply the difference between the actual density of the solid particles and the fluid that is used for separation. For example, if particle A, with a specific gravity of 3.0, were to be separated from particle B, with a specific gravity of 2.0, using a fluid with a specific gravity of 2.5, the resultant density of particle A would be +0.5 and that of particle B would be -0.5. The difference between the resultant densities is the same as that between the actual densities. In the example above the difference in resultant density is also one.
Known gravity recovery machines rely on combinations of the elutriation and sink-float principles under standard gravity. Besides machines named for the specific principle, others that are well known are sluices and jigs. All have been developed to work quite effectively with reasonably large particle sizes and reasonably large material density differences. They reach their limitations with small particles or small density differences.
As settling rates are affected by density or specific gravity, smaller particles may be separated more effectively and accurately in a centrifuge because of the increased difference between their resultant densities and consequent differences in settling rates.
When sink-float separation is performed in a centrifuge, the impelling force is increased because it becomes the resultant vector of centrifugal force and gravity. Differences in resultant density are accentuated accordingly and particle separation becomes more accurate and effective because of that increased difference. With the above particles placed in a centrifuge that applied a combined loading of 10 times gravity the resultant densities would increase proportionately to give a resultant density difference of 10.
Numerous centrifugal devices have been proposed to achieve fine particle separation. Unfortunately, all have their limitations. One of the first of these centrifugal devices was the Ainlay Bowl which in essence is a centrifugal sluice. Pulp material is fed into the center of a spinning bowl-shaped basin from a stationary pipe. That feed is then forced upward and outward along the inner surface by centrifugal force. Horizontal riffles placed on the inner surface collect gold and other dense minerals while the lighter materials are washed up to and discharged over the bowl periphery. Its operational principle is heavy media separation with fluidization provided by the turbulence caused by the pulp transport fluid passing over the riffles. Later adaptations provided additional turbulence with stationary baffles placed near the surface of the riffles to further agitate the transport fluid. Major problems with this device result from the insufficient and irregular fluidization provided by the transport fluid and the fact that the riffles quickly become filled with heavy minerals and cease to function effectively. When that happens, the unit must be shut down and cleaned or emptied, like any other sluice.
A more recent adaptation of the Ainlay Bowl consists of the addition of fluid through small holes through the side of the bowl in the riffle area. This adaptation introduces elutriation to the device and improves both recovery and concentrate quality by improving fluidization in the recovery area of the machine. Two machines that utilise this principle are currently being manufactured and are marketed under the names of the Knelson Bowl and the Falcon Super Bowl.
The above mentioned centrifugal machines are a major improvement over gravity recovery machines. Basically these machines are centrifugal sluices which operate under the principle of elutriation and must be periodically shut down and cleaned. This causes operational problems and essentially restricts their use to the separation of precious metals and other extremely valuable products. Undesirable material particles with high settling rates are recovered along with those that are desirable. Often this causes the riffles to fill too quickly and causes desired materials to be lost. They are also not effective in recovering extremely fine or small sized desirable particles and become ineffective when the density difference between desirable and non-desirable particles falls below a threshold.
U.S. Pat. No. 4,056,464 issued to Cross discloses a jig that utilizes a frusto-conical rotating rotor and screen made of woven wire mesh or perforated sheet metal for the concentration of heavy minerals. The screen is rotated in a chamber containing nominally stationary fluid. It is stated that the pulsations of a plunger along with centrifugal force created by the rotating rotor will cause a vortex to form above the co-axial container (screen) and that heavy minerals will pass through the screen for outside collection. The disclosed frusto-conical screen, as measured from the axis of rotation, would have to be of a low angle in order to create the stated vortex and cause the feed material to move outward and upward. If such a vortex were to form, the actual free surface of the fluid would be in the shape of a paraboloid of revolution about the axis of rotation and material would therefore be unevenly distributed along the surface of such a low angle screen. However, the drawings of the patent indicate that the basket and screen are set at approximately 45 degrees from the vertical axis. This would cause the particles to be subjected to widely varying centrifugal forces as they moved up the screen. Recovered product quality would therefore be virtually impossible to control.
U.S. Pat. No. 4,279,741 issued to Campbell discloses a centrifugal jig with a cylindrical screen. The jig is rotated to produce an outward centrifugal force that is substantially greater than the force of gravity on a slurry supplied to the jig. A liquid is pulsed inwardly from a hutch chamber to produce a fluidic bed within the rotating jig screen to permit settling and separation of the heavy fraction of the slurry from the lightweight fraction.
Campbell discloses that the fluid pulsing apparatus of FIGS. 1 to 5 may be adjusted to allow a selected amount of fluid seepage to the jig bed. Campbell also discloses an arrangement where fluid pulses are provided by rotating a hutch chamber having openings which periodically communicate with openings in a stationary pulsator.
U.S. Pat. No. 4,998,986 also issued to Campbell discloses an improvement to the fluid pulsing system by providing more abrupt shock waves or pressure pulses to the rotating hutch chamber of the jig. To create the pulses, continuously flowing pressurized fluid is alternately directed either to the interior space of the hutch chamber or to the interior space of a surrounding enclosure.
However, I have recognised that for the jig to function, the cylindrical screen would have to be extremely short along its Y or vertical axis, or it would have to be operated at extremely high speeds to generate extremely high centrifugal forces at the screen surface. Otherwise, fluid leaking over a dam would leave an area of the hutch chamber vacant of water and filled with air. That air filled area would then compress during the pulse phase and prevent the jig bed from being agitated.
U.S. Pat. No. 4,898,666 issued to Kelsey discloses a centrifugal jig that utilizes a deep bed of ragging retained by a parabolic shaped screen that separates the slurry supplied region from a hutch chamber. A small amount of water is continuously added to the hutch chamber to replace the water, not more than 5%, lost through the screen, and the bed is pulsed by the hutch chamber water which is driven by a mechanically actuated rubber diaphragm. In a second example, Kelsey discloses actuation of the pulsing diaphragm with compressed gas or by electromagnetic means. The jig has a high number of moving parts that would be difficult to lubricate properly in an environment where water and fine abrasive sand are abundant.
It remains desirable to develop a dependable apparatus that will effectively and continuously separate small sized dense particles from similar sized less dense particles in a pulp material.